Research Article pubs.acs.org/journal/ascecg
Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10206-10214
Platinum Terpyridine Metallopolymer Electrode as Cost-Effective Replacement for Bulk Platinum Catalysts in Oxygen Reduction Reaction and Hydrogen Evolution Reaction Sait Elmas,† Wesley Beelders,† Siobhan J. Bradley,‡ Renee Kroon,§ Geoffry Laufersky,‡ Mats Andersson,†,§ and Thomas Nann*,†,‡ †
Future Industries Institute, University of South Australia, Mawson Lakes Campus, Adelaide, South Australia 5095, Australia The MacDiarmid Institute, School of Chemical and Physical Sciences, Victoria University of Wellington, Laby 410 Gate 6 Kelburn Parade, Kelburn, Wellington 6140, New Zealand § Department of Chemistry and Chemical Engineering, Chalmers University of Technology, 41296 Gothenburg, Sweden
ACS Sustainable Chem. Eng. 2017.5:10206-10214. Downloaded from pubs.acs.org by NEW MEXICO STATE UNIV on 07/01/18. For personal use only.
‡
S Supporting Information *
ABSTRACT: Conducting polymers consisting of metal-selective coordination units and a highly conductive backbone, so-called metallopolymers, are interesting materials exposing single atoms for photo/electrocatalysis and thus represent a potential low-cost alternative for bulk or nanoparticulate platinum group metals (PGMs). We synthesized and fully characterized an electropolymerisable monomer bearing a pendant terpyridine unit for the selective complexation of PGMs. Electrocatalytic tests of the resulting metallopolymer, poly-[(tThTerpy)PtCl]Cl, revealed activity both in the oxygen reduction reaction and hydrogen evolution reaction. Rotating disk experiments showed the direct four-electron reduction of molecular oxygen to water at low angular velocities of the rotating electrode. Furthermore, the fabrication of Pt metallopolymers proved to be simple, nonhazardous and versatile. This proof-of-concept opens up the possibility for developing future low-cost electro- and photocatalysts to replace current systems. KEYWORDS: Oxygen reduction reaction, Hydrogen evolution reaction, Electrocatalyst, Conducting polymer, Terthiophene, Metal complex, Low metal loading, Fuel cells
■
INTRODUCTION
backbone that consists of a less expensive and abundant organic material.11−14 In an economic sense, the bulk inorganic electrode material that is not explicitly involved in catalysis (“buried catalyst material”) can be replaced by organic polymers without affecting the conducting ability of the electrode. Furthermore, the mechanical flexibility of the polymer and the metal selectivity of tailor-made coordination environments can address the major technical hurdle: the leaching of catalyst material during mechanical stress. We designed a monomer consisting of an easily electropolymerisable backbone and a pendant, chelating unit that has a strong binding character to the late transition metals, particularly the platinum group metals. The terpyridine (terpy) ligand is one of the most well-known tridentate pincer-type15−18 and chelating ligands. Its planarity, the donor set N^N^N and the bite-angle tailor the ligand for coordination to a series of late transition metals. Additionally, it can be easily combined with a series of coligands to tailor its activity. The backbone of the ligand is ideally a unit that is easily
The high cost and low natural abundance of platinum group metals (PGMs) is a major obstacle to their broad technical application calling for more sustainable structures/designs/ formulations. Such solutions involve replacement of the PGMs by less expensive and earth abundant elements or reduction of the amount of noble metal to make “every atom count” by using single-site metal centers to maintain or exceed the performance of the bulk metal equivalent. In energy conversion applications such as electrocatalytic water-splitting, the use of platinum and its alloys is gaining traction.1−5 However, the high content of expensive platinum materials impedes the widespread application of this technology. Metallopolymers, conducting polymers doped with metal centers in a welldefined coordination environment, are emerging as smart materials in a broad range of photo/electronic devices as they have the potential to combine the processability of soft organic materials with the redox properties of metals.6−10 In electrocatalytic applications, such as water-splitting and oxygen reduction reaction (ORR), the metallopolymer or metallosupramolecular polymers can therefore provide hybrid functionalities. This makes it possible to embed superior catalytic properties of transition metals into a conducting © 2017 American Chemical Society
Received: July 3, 2017 Revised: August 22, 2017 Published: September 25, 2017 10206
DOI: 10.1021/acssuschemeng.7b02198 ACS Sustainable Chem. Eng. 2017, 5, 10206−10214
Research Article
ACS Sustainable Chemistry & Engineering
processed with CasaXPS (ver. 2.3.16 PR 1.6) data processing software using a Shirley background correction. Single crystals were mounted in paratone-N oil on a plastic loop. Xray diffractions were collected with Mo Kα radiation (λ = 0.7107 Å) on an Oxford Diffraction X-calibur single crystal X-ray diffractometer. Data sets were corrected for absorption using a multiscan method. The structures were solved and refined by direct methods using OLEX software package.27 All non-hydrogen atoms were refined with anistropic displacement parameters. The figures were generated with OLEX. The electropolymerization of the monomer was performed in a three-electrode configured electrochemical cell on an AUTOLAB potentiometer using a gold slide (microscope slide with 100 nm Au and 40 nm Ti sublayer, obtained from Deposition Research Laboratory, Inc.) as working electrode, a platinum mesh (1 × 1 cm, 1 mm Ø) as counter electrode and a silver wire as pseudoreference electrode. The working and counter electrodes were cleaned in a piranha solution (30% H2O2/98% H2SO4 = 1/3 (v/v)) before use. For the oxygen reduction reaction (ORR) experiments, a rotating disc electrode (glassy carbon, 0.197 cm2) was electrochemically deposited with polymer and submerged in a K2[PtCl4] (40 mg in 20 mL H2O) for 1 h at 50 °C. The rotating disc voltammograms were taken on a RSI potentiostat at angular velocities between 200 and 2000 rounds per minute (rpms) using the rotating disc electrode deposited with metallopolymer as working electrode (WE), Ag|AgCl in a saturated KCl solution as reference electrode (RE) and a platinum loop in proton membrane as counter electrode (CE). Hydrogen evolution reactions were performed in 0.1 M KCl solution in an electrochemical cell (EC) using the metallopolymers A and B on Au substrates as working electrode (WE), a Pt rod as counter electrode (CE) and Ag|AgCl (3 M KCl) as reference electrode. Prior to sweeping between 0 and −1.5, the electrolyte solution was saturated with argon gas. After 3−5 potentiometric sweeps, the evolved hydrogen was taken from the headspace of the EC and detected by gas chromatography (SRI Gas Analyzer with thermal detector) using N2 as carrier gas. Synthesis of Poly-[(tThTerpy)PtCl]Cl. 2,5-Dibromo-3-thiophenecarbaldehyde. Following the procedure in the literature,28 a mixture of bromine (2.45 mL, 47.88 mmol) and 7.5 mL of aqueous HBr (48%) was added dropwise to a mixture of 2 mL thiophene-3carboxaldehyde (22.83 mmol) and 10 mL of HBr in 20 mL Et2O at 0 C using an ice bath. The reaction mixture was then refluxed at 50 °C for overnight, cooled to room temperature, quenched with 25 mL of saturated NH4Cl solution (aq) and extracted three times with water. The combined organic phases were washed with brine and dried over anhydrous Na2SO4. All solvents and volatiles were removed on the rotary evaporator and the product was isolated via column chromatography using an eluent mixture of ethyl acetate and cyclohexane in ratio 19/1. After removal of the solvents and volatiles, 5 g of yellow product (95% yield) was obtained. 1H NMR (300 MHz, CDCl3, δ): 10.03 (1H, s, CHO), 7.56 (1H, 1, H4). 13C NMR (75 MHz, CDCl3, δ): 183.6 (1C, CHO), 139.7 (1C, C3), 129.2 (1C, C4), 124.7 (1C, C2), 113.8 (1C, C5). The 1H and 13C APT NMR spectra of the compound are depicted in Figure S2A,B, Supporting Information. 3′-Formyl-2′:2′,5′:2″-terthiophene. To a stirred mixture of 2,5dibromo-3-thiophenecarbaldehyde (5 g, 18.5 mmol) and tetrakis(triphenylphosphinepalladium [Pd(PPh3)4] (1.125 g, 1.11 mmol) in 1,2-dimethoxyerhane (300 mL) were added 2-thiophene boronic acid (5.42g, 42.3 mmol) and a solution of 1 M Na2CO3 (110 mL). The reaction mixture was degassed and flushed three times with argon, and refluxed for 5 h. After that, another portion of 2-thiophene boronic acid was added under protection gas and the mixture was refluxed for another 8 h. The reaction mixture was then cooled down to room temperature and all solvents and volatiles were removed on the rotary evaporator. The crude product was redissolved in CH2Cl2, washed three times with deionized water and the combined organic phases dried over anhydrous Na2SO4. After filtration on a silica pad, the product was obtained as bright yellow solid via column chromatography using ethyl acetate/cyclohexane (1/1) as eluent (3.6 g, 90%). 1H
electropolymerisable onto any electrode surface where the asprepared polymer film provides good conductivity behavior. Among the conductive polymers polythiophene, (PTh) and its derivatives are the most well-studied and applicable candidates.19 They are highly conductive,20,21 easily synthesized and they have already been pursued in applications such as organic solar cells (OSCs),22,23 field-effect transistors24 and electrocatalysis.25,26 Ideally, the as-prepared polymer film is then easily loaded with the metal of choice by dipping the electrode into a solution of the metal salt without any further complicated synthetic routes for complexation (Figure 1A). The electro-
Figure 1. (A) Illustration of the complexing of platinum(II) ions by dipping of the polymer in a solution of potassium tetrachloroplatinate(II). (B) Concept of the platinum-metallopolymer with single-site catalytic centers for the ORR.
chemically synthesized metallopolymer, loaded with platinum(II), was then tested for the electrocatalytic hydrogen evolution reaction (HER) and oxygen reduction reactions (ORR). Figure 1B illustrates the concept of replacing a bulk platinum electrode by a poly(terpyridinyl)terthiophene metallopolymer, poly[(tThTerpy)PtCl]Cl, with distinct platinum(II) catalytic centers for the oxygen reduction reaction (ORR).
■
EXPERIMENTAL SECTION
Materials and Methods. All chemicals and solvents were obtained from Sigma-Aldrich without further purification. All intermediates involving cross-coupling reactions were synthesized under standard Schlenk techniques using argon as protecting gas. The monomer was synthesized under ambient and reflux conditions without further precautions. NMR spectra were recorded with a Bruker Avance II 300 MHz spectrometer by using a triple-resonance 1H, nBB inverse probe head. Unambiguous assignment of the 1H and 13C resonances was achieved from 1H COSY, 13C APT, HSQC and HMBC spectra. All 2D experiments were performed under standard pulse sequences from the Bruker pulse program library. The chemical shifts are quoted relative to TMS. XPS measurements were performed using monochromatized Al Kα X-rays (1486.7 eV) at a power of 225 W on a Kratos Axis-Ultra spectrometer (160 eV analyzer pass energy for survey scans, 20 eV for high-resolution scans). The analysis spot size was ∼300 × 700 μm. Core electron binding energies are given relative to an adventitious hydrocarbon C 1s binding energy of 284.7 eV. All XPS spectra were 10207
DOI: 10.1021/acssuschemeng.7b02198 ACS Sustainable Chem. Eng. 2017, 5, 10206−10214
Research Article
ACS Sustainable Chemistry & Engineering NMR (300 MHz, CDCl3, δ): 10.08 (s, 1H, CHO), 7.56 (s, 1H, H4′), 7.50 (dd, 2H, H5), 7.32 (dd, 2H, H3), 7.29 (dd, 1H, H5″), 7.23 (dd, 1H, H3″), 7.16 (dd, 1H, H4), 7.05 (dd, 1H, H4″). Full 1H NMR spectrum of the compound is depicted in Figure S3, Supporting Information. 4-(terthiophenyl)-Terpyridine.29 2-Acetylpyridine (490 mg, 4 mmol) was added into a solution of 3′-formyl-2′:2′,5′:2″-terthiophene (540 mg, 2 mmol) in EtOH (100 mL). KOH pellets (220 mg, 4 mmol) and NH3 (10 mL, 30%) were then added into the solution. The mixture was stirred overnight. The formed yellow-brown solid was removed and washed two times with cold EtOH/H2O (1/1) leading to 15.6% of yellow product. Alternative synthesis method:30 2acetylpyridine (200 mg, 1.65 mmol) was combined with 3′-formyl2′:2′,5′:2″-terthiophene (204 mg, 0.83 mmol) and KOH pellets (93 mg, 1.65 mmol) and ground for 10 min using a mortar and pestle. The homogeneous crude mixture was then transferred with 50 mL acetic acid into a round-bottomed flask and refluxed with excess of NH4OAc at 110 C for 20 h. The dark red solution was then reduced to half volume on the rotary evaporator and the crude product precipitated with water. After several steps of filtration over silica gel using a series of solvents the product was then isolated via column chromatography in ethyl acetate and cyclohexane (1/6) resulting in yields
